Long-Term Debt - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Series 2014-1 4.277% Fixed Rate Senior Secured Notes, Class A-2
Debt Instrument [Line Items]
Debt interest rate (percent)	4.277%	4.277%
Series 2018-1 Variable Funding Senior Notes Class A-1, at a variable interest rate of 4.93% as of December 31, 2018
Debt Instrument [Line Items]
Debt interest rate (percent)		4.93%
Repayments of notes payable	$ 25.0
Line of credit facility, amount outstanding	$ 25.0
Debt, weighted average interest rate	4.88%
Line of credit facility, current borrowing capacity	$ 222.8
Letter of Credit \| Series 2018-1 Variable Funding Senior Notes Class A-1, at a variable interest rate of 4.93% as of December 31, 2018
Debt Instrument [Line Items]
Debt instrument, collateral amount	$ 2.2

Amount of assets pledged to secure a debt instrument.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Weighted average interest rate of debt outstanding.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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The cash outflow for a borrowing supported by a written promise to pay an obligation.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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